1. Field of the Invention
The present invention relates to an electronic module and a method for manufacturing the same.
2. Description of the Related Art
Currently, in liquid crystal display units and the like, a tape carrier package (TCP) method and a chip on glass (COG) method are mainly used for connection between a display panel and a driver for driving the display panel.
FIG. 1A is a diagrammatic view showing a structure of a conventional TCP type liquid crystal module, and FIG. 1B is a cross-sectional view taken along line A–A′ in FIG. 1A. In this liquid crystal module, as shown in FIG. 1B, a plurality of gate TCPs (driver circuit boards) 12 and source TCPs 14 are connected to a gate terminal region and a source terminal region of a display panel 1, respectively, via corresponding anisotropic conductive films (ACFs) 17. The gate TCPs 12 and the source TCPs 14 are also connected to a gate printed wiring board (PWB) 13 and a source PWB 15, respectively, via corresponding ACFs 17. The gate PWB 13 and the source PWB 15 are soldered to corresponding flexible printed circuits (FPCs) 16 that are connected to an external circuit board. In the TCP type liquid crystal module described above, signals and the power supply voltage (hereinafter, these are sometimes collectively called “signals” for simplification) are directly supplied to all of the gate TCPs 12 and all of the source TCPs 14 from the gate PWB 13 and the source PWB 15, respectively.
A substrate-free type liquid crystal module that eliminates the need for the PWBs provided for the TCP type liquid crystal module described above has been proposed.
For example, Japanese Laid-Open Patent Publication No. 2001-188246 (Literature 1, FIG. 9) discloses a substrate-free structure, in which, as shown in FIG. 2, panel wiring lines 18 and 19 run on a gate terminal region and a source terminal region of a display panel 11, respectively. A signal input via a FPC 16 is transferred to a plurality of gate TCPs 12 or source TCPs 14 sequentially from one TCP to its adjacent TCP via the panel wiring line 18 or 19.
FIG. 3A is a diagrammatic view showing a structure of a COG type liquid crystal module, and FIG. 3B is a cross-sectional view taken along line A–A′ in FIG. 3A.
As shown in FIG. 3A, in the COG type liquid crystal module, a plurality of source driver ICs 24 (hereinafter, a driver IC is simply called an IC) and a source FPC are mounted on a source terminal region of a display panel 11. Likewise, a plurality of gate ICs 22 and a gate FPC 23 are mounted on a gate terminal region of the display panel 11. As shown in FIG. 3B, the gate ICs 22 and the source ICs 24 are connected to the display panel 11 via corresponding ACFs 17, and also the gate FPC 23 and the source FPC 25 are connected to the display panel 11 via corresponding ACFs 17.
COG type liquid crystal modules have a problem in that the frame width (width of the terminal region) of the display panel 11 is large compared with TCP type liquid crystal modules. A liquid crystal module for solving this problem is disclosed in Japanese Laid-Open Patent Publication No. 2000-137445 (Literature 2, FIG. 1), for example. FIG. 4 is a partial diagrammatic view of a liquid crystal module disclosed in Literature 2. This liquid crystal module includes a FPC 25 having a comb shape, which has portions protruding into spaces between adjacent source ICs 24. Each of the source ICs 24 is connected to the FPC 25 at both sides via terminals 29 on the display panel. Having this structure, the frame width of this liquid crystal module is smaller than that of the liquid crystal module shown in FIGS. 3A and 3B.
The conventional liquid crystal modules described above have the following problems. The TCP type liquid crystal module shown in FIGS. 1A and 1B has a long track record in mass production, but has a problem that the number of components is large and the manufacturing cost including materials cost and mounting machining cost is high. This liquid crystal module also has a shortcoming in that the module assembly work is complicated due to the large PWBs extending from the display panel 11.
On the contrary, the substrate-free liquid crystal module shown in FIG. 2 has a small number of components compared with the TCP type liquid crystal module described above, and thus, the manufacturing cost can be reduced. In this type of liquid crystal module, in which a signal is transferred to the plurality of TCPs from the input FPC 16, wiring impedance on the display panel 11 must be held to a low level. However, since the panel wiring lines 18 and 19 are normally formed by a thin film deposition process, the impedance thereof is considerably high compared with wiring lines formed on the TCPs. Therefore, if the adjacent TCPs are spaced farther apart from each other due to an increase in size of the display panel 11, thus the panel wiring lines 18 and 19 become longer, and as a result, the influence of the impedance on the display quality will be significant. Also, if the number of TCPs increases due to high definition of the display panel 11, the number of TCPs to which a signal is transferred from one FPC increases. In this case, also, the display quality will be adversely influenced. The number of FPCs used is generally determined depending on the size of the display panel. For example, two FPCs are provided for VGA class, three FPCs for XGA class, and four or more FPCs for UXGA class.
As described above, in the TCP type and substrate-free type liquid crystal modules, it is difficult to achieve both a reduction in manufacturing cost and high definition, particularly for large-size models.
The COG type shown in FIGS. 3A and 3B has a small number of components and thus can reduce the manufacturing cost, but has a problem in that the frame width of the display panel 11 is large. Increase of the frame width is critical when the liquid crystal module is applied to a notebook personal computer. Even when applied to a device that has comparatively few limitations on the frame width, such as a monitor and a TV set, an increase of the size of the display panel 11 is undesirable because this leads to a cost increase.
The COG type liquid crystal module shown in FIG. 4 has a smaller frame width than the COG type liquid crystal module shown in FIGS. 3A and 3B. However, the process of connecting the ICs 24 to the FPC 25 is complicated, and thus the mounting cost increases.
Moreover, in the COG type liquid crystal modules shown in FIGS. 3A and 3B and FIG. 4, the connection pitch of output terminals of the ICs 22 and 24 for outputting signals to the display panel 11 is narrow compared with that in the TCP type. Therefore, when the limitation (minimum value) of the signal line wiring pitch on the display panel 11 is the same, the run length of signal lines before reaching a display pixel region is larger and thus the distance between the ICs 22 and 24 and the display pixel region is longer, than in the TCP type. For this reason, also, the frame width increases.